1. Field of the Invention
The present invention relates to a technology for inspecting an appearance of samples using a scanning electron microscope.
2. Description of the Related Art
Semiconductor devices such as memories and microcomputers for use in computers and the like are manufactured by repeatedly performing a process of transferring patterns of circuits and the like formed on photomasks through exposure treatment, lithography process, etching process, and the like. In a manufacturing process of semiconductor devices, determination results of quality of the lithography and etching processes and other processes and presence of defects such as foreign matter generation significantly affect manufacturing yields of semiconductor devices. Therefore, patterns on semiconductor wafers are inspected when each manufacturing process is completed to detect early or in advance an occurrence of abnormalities or defects.
Images of high SN need to be obtained at very high speed to carry out a high-throughput and high-precision inspection so as to keep pace with increase in wafer diameters and miniaturization of circuit patterns. For this purpose, the number of irradiated electrons is secured by using large-current beams 1000 times or more that of a common scanning electron microscope (SEM), for example, 100 nA or more to maintain the high SN ratio. Further, high-speed and high-efficiency detection of secondary electrons and backscattered electrons generated by the substrate is required.
Moreover, low accelerated electron beams of 2 keV or less are used so that semiconductor substrates having an insulating film such as a resist should not be affected by being charged. This technology is described in Editor: No. 132 Committee of Japan Society for the Promotion of Science, “Electron/Ion Beam Handbook”, 2nd ed., The Nikkan Kogyo Shimbun. Ltd., 1986, pp. 622-623. However, a high-resolution observation is made very difficult by an aberration due to a space charge effect caused by a large-current and low accelerated electron beam.
As a method to solve this problem, a technique to decelerate a highly accelerated electron beam immediately before a sample and to practically irradiate the sample with a low accelerated electron beam is known. For example, a technology described in Japanese Patent Application Laid-Open No. 2-142045 and Japanese Patent Application Laid-Open No. 6-139985 is known.
An inspection system using a scanning electron microscope as described above has a problem described below. When sufficient contrast cannot be obtained due to an insufficient amount of irradiation of an electron beam irradiated on a sample, it is effective to lower the sampling rate from the standard level. However, if an attempt is made to lower the sampling rate significantly, an AD converter element performing digital conversion may not be able to handle beyond the operating range. High-precision AD conversion is needed to solve this problem. A technology to cause a low-resolution AD converter to operate as a high-resolution AD converter by performing digital conversion of each piece of continuously sampled data before addition thereof is described in Japanese Patent Application Laid-Open No. 05-55919.
Japanese Patent Application Laid-Open No. 02-142045
Japanese Patent Application Laid-Open No. 06-139985
Japanese Patent Application Laid-Open No. 09-288989
Japanese Patent Application Laid-Open No. 2005-251753
Japanese Patent Application Laid-Open No. 2005-251754
Japanese Patent Application Laid-Open No. 05-55919
Editor: No. 132 Committee of Japan Society for the Promotion of Science, “Electron/Ion Beam Handbook”, 2nd ed., The Nikkan Kogyo Shimbun. Ltd., 1986, pp. 622-623